The present invention relates to a leakage current reduction circuit and a power supply employing the leakage current reduction circuit, and in particular, to a leakage current reduction circuit capable of reducing leakage current of a switching power supply.
AC adapters are widely used as power supplies of electronic devices, especially for portable electronic devices such as notebook computers. An AC adapter generally includes a Y capacitor as a component for preventing EMI (Electro-Magnetic Interference). The electric shock problem of electronic devices is mainly attributed to two factors: leakage current and input voltage. If we assume that the input voltage is set constant, the leakage current is determined by the capacitance of the Y capacitor. When the capacitance of the Y capacitor is decreased, the leakage current becomes smaller and thereby the electric shock to the user of the electronic device also becomes smaller. In order to reduce the leakage current to a level at which the electric shock is negligible, the capacitance of the Y capacitor has to be reduced to a very low level, while the prevention of the EMI becomes incomplete if the capacitance is reduced to the low level. Therefore, under the present situation, the capacitance of the Y capacitor can not be reduced enough for the reduction of the leakage current and the electric shock problem.
FIG. 1 is a circuit diagram showing an example of a conventional AC adapter employing a Y capacitor. The AC adapter 104A shown in FIG. 1, which is implemented as a switching power supply of flyback type (flyback converter), includes a rectifier 41 (which is composed of a diode bridge), a smoothing capacitor 42, a transformer 43, a rectifier 44 (which is composed of a diode), a smoothing capacitor 45, a controller 46, a switching element 47, and a Y capacitor 5. Commercial AC power from an AC socket 6 is supplied to the AC adapter 104A through an AC cable 7.
The AC input is rectified by the rectifier 41 and the rectified output of the rectifier 41 is smoothed by the smoothing capacitor 42, thereby conversion of the input AC voltage to a DC voltage is conducted first. The DC voltage is converted again to an AC voltage by on-off action of the switching element 47, and the AC voltage is supplied to the primary coil of the transformer 43. The on-off action of the switching element 47 is controlled by the controller 46.
An AC voltage obtained at the secondary coil of the transformer 43 is converted again to a DC voltage by the rectifier 44 and the smoothing capacitor 45. The DC voltage obtained by the above operation is supplied to a notebook computer 9 through a DC output cord 8. The Y capacitor 5 for the prevention of the EMI is inserted between a grounding line 13 on the secondary side of the transformer 43 and the midpoint of the rectifier 41 on the primary side of the transformer 43.
In every electronic device, a component equivalent to the Y capacitor 5 causes leakage current by nature. Therefore, when the voltage of the AC socket 6 is AC 120V, voltage difference of approximately AC 60V appears between metal part 11 of the notebook computer 9 and a ground 16, thereby a leakage current which is proportional to the capacitance of the Y capacitor 5 and which depends on the resistance between the metal part 11 and the ground 16 passes between the metal part 11 and the ground 16 (that is, passes through the user of the notebook computer 9). When the user (equivalent resistance 12 shown in FIG. 1) is touching the metal part 11 of the notebook computer 9, there is a possibility that a leakage current of approximately 100 xcexcA passes through the user. The leakage current of such a low level is safe for the user, however, there exist rare cases where the user feels an electric shock.
FIG. 2 is a circuit diagram showing another example of a conventional AC adapter. The AC adapter 104B shown in FIG. 2 is a switching power supply of the flyback type (flyback converter), further including two Y capacitors. Also in the example of FIG. 2, voltage difference of approximately AC 60V appears between the metal part 11 of the notebook computer 9 and the ground 16, and there is a possibility that the user touching the metal part 11 feels an electric shock.
FIG. 3 is a circuit diagram showing another example of a conventional AC adapter. The AC adapter 104C shown in FIG. 3 is a switching power supply of RCC type (RCC converter). The AC adapter 104C of FIG. 3 has different composition on the primary side of the transformer 43, in comparison with the conventional flyback-type AC adapter 104A of FIG. 1.
FIG. 4 is a circuit diagram showing another example of a conventional AC adapter. The AC adapter 104D shown in FIG. 4 is a switching power supply of the RCC type (RCC converter), further including two Y capacitors.
FIG. 5 is a circuit diagram showing another example of a conventional AC adapter. The AC adapter 104E shown in FIG. 5 is a switching power supply of forward type (forward converter). The AC adapter 104E of FIG. 5 has different composition on the secondary side of the transformer 43, in comparison with the conventional flyback-type AC adapter 104A of FIG. 1.
FIG. 6 is a circuit diagram showing another example of a conventional AC adapter. The AC adapter 104F shown in FIG. 6 is a switching power supply of the forward type (forward converter), further including two Y capacitors.
Also in the examples of FIGS. 3 through 6, voltage difference of approximately AC 60V appears between the metal part 11 of the notebook computer 9 and the ground 16 and there is a possibility that the user touching the metal part 11 feels an electric shock.
The maximum permissible level of the leakage current has been determined by UL (Underwriters Laboratories Inc.) etc., and leakage current below the maximum permissible level is generally regarded as safe. However, there have been some reports in recent years that electric shocks are felt by some users even if the leakage current is within the maximum permissible level. Therefore, the reduction or elimination of the electric shock without sacrificing the EMI prevention capability is required today.
For meeting the request, grounding by use of 3-terminal AC input have been generally employed. FIG. 7 is a circuit diagram showing an example of a conventional flyback-type AC adapter employing the 3-terminal AC input and the grounding, in which the same reference characters as those of FIG. 1 designate the same or corresponding parts to those of FIG. 1 and thus repeated description thereof is omitted for brevity. In the example shown in FIG. 7, the AC socket 6 is composed of three terminals including a GND (grounding) terminal which is grounded. The electric shock can be eliminated by connecting a GND (grounding) terminal of the AC adapter 104G to the GND terminal of the AC socket 6 by use of a grounding wire.
However, the AC sockets 6 employed in ordinary houses and office buildings have 2-terminal structure in most cases, and thus the grounding to the GND terminal is difficult. Even if the AC socket 6 is provided with a GND terminal, portable electronic devices (notebook computers etc.), which are supposed to be carried freely, can not be connected to the GND terminal by use of the grounding wire constantly.
On the other hand, if a portable electronic device is always required to be connected to the GND terminal by use of the grounding wire, portability has to be sacrificed and the advantage and commercial value of the portable electronic device are necessitated to be impaired.
It is therefore the primary object of the present invention to provide a leakage current reduction circuit and a power supply employing the leakage current reduction circuit, by which the leakage current and the electric shock problem of an electronic device employing the leakage current reduction circuit or the power supply can be reduced without deteriorating the portability of the electronic device and the EMI prevention capability.
In accordance with a first aspect of the present invention, there is provided a leakage current reduction circuit for a power supply which converts an input AC voltage to a DC voltage, supplies the DC voltage to the primary side of a transformer intermittently, converts an AC voltage obtained on the secondary side of the transformer to a DC voltage, and outputs the obtained DC voltage. The leakage current reduction circuit includes a first capacitor, a second capacitor and a third capacitor. The first capacitor and the second capacitor are connected in series between two AC input lines which are connected to an input AC cable of the power supply so that the input AC voltage will be divided. The third capacitor for the prevention of EMI (Electro-Magnetic Interference) is provided between a grounding line on the secondary side of the transformer and the connection point between the first capacitor and the second capacitor. The capacitance of the second capacitor is set larger than the capacitance of the first capacitor and the second capacitor having the larger capacitance is connected to one of the two AC input lines that is connected to a neutral line of the input AC cable.
In accordance with a second aspect of the present invention, in the first aspect, capacitance ratio between the first capacitor and the second capacitor is set to approximately 1:10.
In accordance with a third aspect of the present invention, in the first aspect, the leakage current reduction circuit is employed for a flyback-type power supply.
In accordance with a fourth aspect of the present invention, in the first aspect, the leakage current reduction circuit is employed for an RCC-type power supply.
In accordance with a fifth aspect of the present invention, in the first aspect, the leakage current reduction circuit is employed for a forward-type power supply.
In accordance with a sixth aspect of the present invention, in the first aspect, the leakage current reduction circuit is employed for a power supply for a portable electronic device.
In accordance with a seventh aspect of the present invention, there is provided a power supply which converts an input AC voltage to a DC voltage, supplies the DC voltage to the primary side of a transformer intermittently, converts an AC voltage obtained on the secondary side of the transformer to a DC voltage, and outputs the obtained DC voltage, comprising: a first capacitor and a second capacitor which are connected in series between two AC input lines which are connected to an input AC cable of the power supply so that the input AC voltage will be divided; and a third capacitor for the prevention of EMI (Electro-Magnetic Interference) which is provided between a grounding line on the secondary side of the transformer and the connection point between the first capacitor and the second capacitor. The capacitance of the second capacitor is set larger than the capacitance of the first capacitor and the second capacitor having the larger capacitance is connected to one of the two AC input lines that is connected to a neutral line of the input AC cable.
In accordance with an eighth aspect of the present invention, in the seventh aspect, capacitance ratio between the first capacitor and the second capacitor is set to approximately 1:10.
In accordance with a ninth aspect of the present invention, in the seventh aspect, the power supply is implemented as a flyback-type power supply.
In accordance with a tenth aspect of the present invention, in the seventh aspect, the power supply is implemented as an RCC-type power supply.
In accordance with an eleventh aspect of the present invention, in the seventh aspect, the power supply is implemented as a forward-type power supply.
In accordance with a twelfth aspect of the present invention, in the seventh aspect, the power supply is employed as a power supply for a portable electronic device.
In accordance with a thirteenth aspect of the present invention, in the seventh aspect, the input AC cable of the power supply is provided with a mark for discriminating between neutral and hot so that the second capacitor having the larger capacitance will be connected to a neutral terminal of an AC socket correctly.
In accordance with a fourteenth aspect of the present invention, in the seventh aspect, a plug at the end of the input AC cable of the power supply is provided with a mark for discriminating between neutral and hot so that the second capacitor having the larger capacitance will be connected to a neutral terminal of an AC socket correctly.